Roof Replacement and
Roof Repair Specialist(703) 475-2446
In this excerpt, you’ll learn how to calculate the quantity of shingles you’ll need for a roofing job. I’ll describe two methods for determining the area of a roof and how to gauge the number of extra shingles you’ll need for waste, overlaps, and starter shingles.
Figuring out the roof area is the first step to determine how many bundles you’ll need to order. There are two ways to size up a new or freshly stripped roof: the measurement method and the sheet-count method. There’s a third method for calculating bundles when you’ll be laying new shingles over old or if the old roof is still in place.
Once you have a bundle or square count for the main roof area, you’ll add additional shingles to account for waste, starter shingles, and extra shingles for hip and ridge caps.
Each structural panel is 32 sq. ft., and you can easily count the full panels from the ground. You can also tally them up by gauging the relative size of ripped and crosscut sheets along the edges of the roof to the size of a full sheet. Diagonally cut sheets along hips and valleys are a little more difficult to size, but you can usually assign them a relative size such as half or quarter of a full sheet and be close enough (see the accompanying drawing).
If the shingles you are using come three bundles to a square, calculating the number of bundles you’ll need is simple. Each bundle covers 33.3 sq. ft. of roof area—close enough to the 32 sq. ft. a sheet covers. So just order one bundle for each sheet of roof sheathing.
For other bundle counts per square, divide the number of sheets of sheathing by three and you’ll have the total number of squares needed to cover the roof. This works because three sheets of sheathing equal roughly 100 sq. ft. (one square).
This may sound like a crude measurement method, but you have to be realistic about how accurate you really need to be. The 1.3-sq.-ft. difference per sheet is a good margin to allow for waste along gable or hip edges. I’ll look at calculating waste further in the next section.
First, measure the length of the eaves of each roof plane, either directly or from the ground by measuring the length of the house and adding in the width of the rake overhangs, if any. Alternately, if the existing shingles are standard three-tab, you can get the eaves’ length by counting the number of tabs along the ridges and eaves to determine the length in feet (one tab is equal to 1 ft.).
Now, to get the length of the rakes, count the existing courses of shingles from eaves to ridge. The exposure on each course of shingles is 5 in., so you can multiply the number of courses by 5 in. and then divide by 12 to get the length of the rakes. Just be sure to check that the existing shingles are standard 12-in. by 36-in. shingles and not metric size. Multiply the length of the eaves by the length of the rake and you have the area in square feet.
For a simple roof, I generally figure 1 percent as a waste factor. On a complex roof with open valleys, I add 5 percent and sometimes more. There’s no calculation you can use to determine the extra shingles you’ll need for waste. With experience estimating jobs, you’ll get a feel for how many extra shingles to order.
Waste is also generated when shingles are damaged, which is inevitable when you’re moving shingles around a steep roof slope and some will slide off. You may be able to salvage part of the damaged shingle, but don’t count on it. You can also waste shingles when you nail them improperly and have to remove them. You may drive nails too low in the exposure or fasten one off a control line.
If you have a crew that tends to be sloppy and damage shingles, you’ll probably need a couple of extra bundles per job. If your crew is conscientious, one extra bundle should be enough for most jobs.
When you’re using laminated shingles to roof a house, you can save a little money by ordering cheaper three-tab shingles for the starters or better yet, use shingles left over from a previous job. Since laminated shingles have no cutouts, all you really need the starter strip to do is seal the shingles down and shed any water that passes through the joints between shingles.
On roofs shingled with three-tab or no-cutout shingles, you simply cut standard shingles into caps. You can cap about 35 lin. ft. of ridge or hips with each bundle of three-tab shingles that come three bundles to the square. You can also salvage waste shingle pieces and portions of damaged shingles for use as caps.
For roofs shingled with laminated shingles, multi-cutout shingles, and other patterned shingles, you’ll have to order hip-and-ridge shingles that are manufactured as companions to the specific shingle product you are using. They’re sold by the bundle and usually cap 35 lin. ft., but check with your supplier because some products vary. Just measure the length of the ridges and hips and divide by 35 to determine how many bundles of regular or hip-and-ridge shingles you’ll need.
Areas of complex roofs with multiple hips and valleys take the most time to calculate. Start by making a rough sketch of the roof. To simplify the calculation, break down the sketch into rectangles and right triangles (triangles with one 90-degree corner), then take as many measurements of the roof as you can to match the sides of the rectangles and triangles on the sketch.
Use visual cues from the existing roof shingles or roof sheathing to determine square lines off eaves edges or ridges. These cues will help you measure the lengths of the sides of the rectangles and triangles. For instance, the cutout slots on shingled roofs run perpendicular (90 degrees) to the eaves, and nail rows in sheathing are pretty close to square also. It is difficult sometimes to get accurate measurements. Don’t get too concerned though; just round lengths to the nearest 6 in.
With the sketch filled in with measurements, you can determine the size of the roof area. The area of a rectangle is length multiplied by width, whereas the area of a right triangle is the length of the two sides that meet at the 90-degree corner multiplied together and divided by two (this works because a right triangle is half a rectangle).
Tally the square footages of all the rectangles and triangles, which will give you the total square footage for the roof. The example here shows the calculation for a roof with two hips.
About the Author:
Fine Homebuilding Magazine
Reprinted with permission © 2002, The Taunton Press, Inc.